| Laser additive manufacturing technique has several significant advantages over traditional manufacturing methods,such as a wide range of material forms,a simple manufacturing process for complex parts,the parts with the excellent performances,etc.,which is an effective method for the personal customization of biological implants.It shows a broad application prospect in the preparation of the biological implants.It is well documented that the phase transformation of the alloys during the the laser additive manufacturing is affected by the multi-field coupling action such as temperature and stress,and thus its process is very complicated,leading to the various morphologies of the microstructure of the alloy which are sensitive to the compositions and the process parameters.So,the study of the effects of the process parameters and the heat treatment of the microstructure and mechanical properties of the alloy prepared by laser additive manufacturing is a research hotspot.Therefore,in the present thesis,a ?-type Ti-26 Nb alloy with low elastic modulus,good corrosion resistance and biocompatibility was taken as a research object,and the Ti–26Nb alloy was elaborated by laser additive manufacturing of coaxial powder feeding with Ti and Nb mixed powders as raw materials.The effects of process parameters and subsequent heat treatment on the microstructure and mechanical properties of as-deposited Ti-26 Nb alloy fabricated by laser additive were investigated.Moreover the anisotropy and the influence factors on it of the deposited Ti-26 Nb alloy were discussed.The main content and the results are as follows:(1)The effects of the process parameters on the the microstructure and properties of the as-deposited Ti-26 Nb alloy were investigated.It is found that,at conditions of laser nominal output power of 750 W,scanning speed of 480 mm/min and powder feed rate of 2.2 g/min,the Ti-26 Nb alloy with high quality can be obtained.The microstructure of the as-deposited Ti–26Nb alloy is characterized by the prior columnar ? grains with a relatively strong <001> fiber texture due to a large temperature gradient and remelting penetration in the build direction.The as-deposited alloy exhibits very high strength with the ultimate tensile strength of about 799 MPa and the yield strength of about 768 MPa.(2)The as-deposited were isothermally annealed at different temperatures,and the effects of the annealing temperature on the the microstructure and the mechanical properties were investigated.The results indicate that the annealing treatment at the temperature ranging from 650 °C to 925 °C can promote the dissolution of the unmelted Nb particles into the Ti lattice,leading to an increase in Nb concentration in the ? matrix,and uniformity of the composition of the alloy by the diffusion of Nb atoms.While,the annealing treatment also leads to the growth in the size of the grains.As a result,the annealing treatment decreases the strength but increases the ductility of the as-deposited alloy.The alloy annealed at 850 °C for 0.5 h exhibits a good balance of strength and ductility with the ultimate tensile strength of 704 MPa and elongation of 21.6%.(3)The microstructure and mechanical properties of the as-deposited Ti-26 Nb alloy show some anisotropy,evidenced by the difference of the tensile properties of the alloy in the lateral direction and in the build direction.The tensile strength and elongation in the lateral direction are 934 MPa and 10.1%,respectively,while,in the in the build direction,they are 799 MPa and 14.3%,respectively.Annealing treatment can reduce the difference of tensile properties in the transverse direction and in the scanning direction,but has a slight influnce on the anistropy of the tensile properties in the horizontal direction and in the vertical direction.Isothermal treatment can effectively improve the uniformity of the composition of the alloy. |
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